1. Field of the Invention
This invention relates to a composite in which a film mainly comprising amorphous titanium oxide is formed on a surface of a base and to a manufacturing method therefor.
2. Description of the Prior Art
It has been known that a base surface is coated with titanium oxide and photoexcited to make the surface hydrophilic. In the prior art, among titanium oxides, only anatase-type titanium dioxide has been made adequately hydrophilic by photoexcitation.
Formation of anatase-type titanium dioxide films requires a high temperature process, which has made it difficult to apply such a film to a base comprising a low melting temperature material such as synthetic resin. For the base made of soda-glass, a high temperature process causes diffusion of sodium in the soda glass into the titanium dioxide film, leading to deteriorated performance (reduction in photoexcitation efficiency). For preventing performance deterioration, it is necessary to form a passivation layer between the soda-glass base and the titanium dioxide film, leading to a more complicated manufacturing method.
In view of these problems, an objective of this invention is to provide a composite in which a novel amorphous titanium oxide film is deposited on a base surface for hydrophilicity and a manufacturing method therefor.
This invention provides a composite comprising a base and a film mainly formed of amorphous titanium oxide on the base, wherein the amorphous titanium oxide partially has a structure in which a network of Tixe2x80x94Oxe2x80x94Ti bond is broken to give Tixe2x80x94OH bond terminals. In general, it is known that amorphous titanium oxide films have photoexcitation efficiency too low to give adequate hydrophilicity. In contrast, our experiments showed that more hydrophilicity could be achieved by photoexcitation using the amorphous titanium oxide film of this invention partially comprising a structure in which a network of Tixe2x80x94Oxe2x80x94Ti bond is broken to give Tixe2x80x94OH bond terminals. It may be because in a common amorphous titanium oxide film, electrons and holes generated by photoexcitation might have a high probability of being recombined due to, for example, the presence of dangling bonds in the film, leading to reduced photoexcitation efficiency, while in a titanium oxide film according to this invention, dangling bonds in the film might be bound to OH groups to reduce the number of dangling bonds so that recombination of electrons and holes generated by photoexcitation might be inhibited and, furthermore, electron-releasing property of an OH group itself might contribute to improvement in hydrophilicity.
In amorphous titanium oxide in this invention, a proportion may widely vary for Tixe2x80x94OH terminals generated by bond cleavage in Tixe2x80x94O bonds in a Tixe2x80x94Oxe2x80x94Ti bond network. If the proportion is less than 5%, hydrophilicity will become too low while if the proportion is more than 30%, film strength will be reduced. Thus, the most suitable proportion is about 5 to 30%. A base may be made of, for example, glass or synthetic resin. When a base is used as a mirror such as an exterior rear view mirror for an automobile and a mirror for a bath room, the mirror may comprise a base made of a transparent material such as glass and synthetic resin; a transparent film; and a reflecting film formed on the rear surface of the base. In another configuration, a film may be transparent and a reflecting film may be formed between a base made of, e.g., glass or synthetic resin and the transparent film.
A composite according to this invention may be manufactured by a method wherein a film in the composite is formed by a plasma CVD technique, comprising the steps of placing a base in a vessel, generating plasma in reduced-pressure oxygen atmosphere in the vessel, and introducing a titanium-containing material in the region where excited species attributed to the plasma exist to deposit a film mainly comprising amorphous titanium oxide on a surface of the base. According to the method, a film mainly comprising amorphous titanium oxide may be deposited in a relatively-low temperature process, allowing use of a material with a low melting point such as synthetic resin as a base. Furthermore, even when using soda glass as a base, a film may be formed in a relatively-low temperature process so that diffusion of sodium in the soda glass into the film may be inhibited, resulting in elimination of the need for a passivation layer and achievement of a simplified manufacturing method. When a content of OH groups in a film is relatively large, a refractive index is reduced as compared with crystalline titanium dioxide, allowing reduction in surface reflection of the film. It may also reduce deterioration in visibility due to a double image which is cased when a mirror is fabricated using the composite of this invention.
In the manufacturing method according to this invention, the titanium-containing material may be, for example, one mainly comprising titanium alkoxide. Such a titanium alkoxide material may mainly comprise a material selected from the group consisting of titanium tetraisopropoxide {Ti(OC3H7)4}, titanium tetraethoxide {Ti(OC2H5)4}, and titanium tetrabutoxide {Ti(OC4H9)4}. During deposition of the film on the base surface, the base may be, for example, at room temperature to 150xc2x0 C. The film may be deposited while allowing ions in the plasma to collide with the base, to effectively decompose a stable monomer such as titanium alkoxide.